[eddyb]

IIRC, that "optimization" mostly avoids wasting time compiling a complex `Clone` implementation, when simply returning `*self` suffices (there are some crates with at lot of `#[derive(Copy, Clone)]` types). We try to avoid having a lot of logic like that too early, for precisely the reasons you mention.

I'd be interested in an example where LLVM can't optimize the general version, as it means we might want to do this through MIR shims instead (which can be generated when collecting the monomorphic instances to codegen - this is what happens when you clone a tuple or closure, for example).